Glossary: Thermal Imaging Systems

Term related to Thermal Imaging

Universal term

Absolute Temperature Scale

Thermodynamic temperature scale, named after Lord Kelvin, in which temperatures are given in Kelvin’s (K). Absolute zero (zero Kelvin) corresponds to -273.15°C or -459.7°F. The size of the Kelvin unit is the same as that of the Celsius degree.

Ambient Temperature

The prevailing temperature in the immediate vicinity of the object; the temperature of its environment.

Aperture

An opening or hole through which radiation must pass.

Area of Interest (AOI)

Also known as a region of interest. The area of an image where some calculation occurs such as calculating the average, maximum or minimum temperature.

Array

Multiples of light-sensitive or infrared sensitive elements in cameras, detector or scanning devices.

Blob

A group of adjacent pixels in an image representing the same value, as all black in a binary image.

Bolometer

A very sensitive thermometric instrument used for the detection and measurement of radiant energy. Its essential component is a short narrow strip covered with a black absorbing coating and mounted at the lower end of a long cylindrical tube having a stop across it to eliminate unwanted radiation. The electrical resistance of the strip changes with the changes in temperature that arise from absorbing varying amounts of radiant energy.

Charged Coupled Device (CCD)

A self-scanning semiconductor imaging device that utilizes MOS (Metal Oxide Semiconductor) technology, surface storage and information transfer. It generally consists of a metal insulator semiconductor (MIS) capacitor, majority carriers being attracted to the semiconductor insulator interface when a negative voltage is applied to the metal. Reversal of the voltage polarity creates a region depleted of majority carriers, an empty potential well. Minority carrier charge representing information accumulates in the well, partially filling it. Information is transferred from one well to another.

- Identification: The most detailed level of description of a particular object within a functional category (unarmed male civilian, four-door truck).

Based on Johnson’s criteria table, 3 pixels are needed to detect an object, 6 to recognize, and 12 to identify it. This approach gives a 50% probability to successfully accomplish a task of object detection/recognition/identification and corresponds to maximum DRI values. To increase said probability to 90%, the number of pixels needs to be increased by 1.8 times, and more specifically to 5.4 pixels for detection, 10.8 for recognition and 21.6 for identification. DRI values listed in the device’s specifications are generally given for reference only and may not correlate with results obtained in real-world conditions, as in some cases many more pixels need to be distinctively seen to successfully detect/recognize/identify an object. This in turn will significantly impact the distances at which an object is detected/recognized/identified.

F-Number or F-Stop

The ratio of focal length to lens aperture (the optical diameter of the lens that lets incoming energy impinge the FPA). The smaller the f- number - the larger the lens diameter, which means a brighter image with a narrower depth-of-field.

Field-of-View (FOV)

The 2D area which can be seen through the optical imaging system.

Focal Plane

A plane (through the focal point) at right angles to the principal axis of a lens or mirror - the surface on which the best image is formed.

Focal Plane Array (FPA)

A linear or two-dimensional matrix of individual detector elements, typically used at the focus of an imaging system.

Forward Looking Infrared (FLIR)

A device that uses one or more infrared transducers to scan a scene in the 3-5 micron or 8-12 micron wavelengths, converts the infrared radiation to electronic data and presents the resulting image on a TV-like screen. The term was originally referred to airborne systems but now is used for any real time thermal imaging systems.

Framerate / Frequency

Also known as Refresh Rate. The number of times per second that the frame of an image is scanned/refreshed. It is expressed in Hertz (Hz) or frames per second (fps).

Gain (Thermal Imaging)

Used to optimize your device’s performance according to environmental conditions. Not to be confused with Gain control for Night Vision systems, Thermal Gain increases or decreases the sensitivity of the FPA to better display the contrasting temperatures of the environment. For example, environments where all objects are of a similar temperature require an increased FPA sensitivity (gain) to produce the best image quality.

Histogram

A graphic representation of a distribution function such as frequency by means of rectangles whose widths represent the intervals into which the observed values range is divided and whose heights represent the number of observations occurring in each interval.

Image Subtraction

A method used to compare two pictures of the same subject taken at different points in time.

Infrared Photoconductor

A photoconductor that demonstrates increased conductivity during its exposure to infrared radiation.

Machine Vision

Interpretation of an image of an object or scene through the use of optical non-contact sensing mechanisms for the purpose of obtaining information and/or controlling machines or processes.

Microbolometer

A camera, individual pixels receive different amounts of thermal radiation from the scene and heat up different amounts with respect to the bolometer array substrate. The part of each bolometer that heats up is thermally isolated from the substrate so that a very small amount of input IR power results in a measurable change in the bolometer's temperature. The amount of heating of each pixel (and thus the intensity of the IR scene) is determined by passing a known current or applying a voltage through a resistive element in the thermally isolated section of the bolometer.

Non-uniformity Correction (NUC)

One of the less desirable characteristics of modern FPA detectors is their relative non-uniformity from detector to detector. This results from variations in the manufacturing process and the detector material itself. The fact remains that all FPA detectors are fairly non-uniform in their response to temperature when they are built. To correct for this, virtually all FPA cameras have some type of non-uniformity correction built into the camera. Methods for correcting this problem vary greatly from manufacturer to manufacturer. The simplest approach is when a lens cap is placed on the camera and a "NUC" button is depressed and the camera corrects for uniformity based on the temperature of the lens cap. Other systems have a uniform temperature "paddle" within the camera which is inserted in the optical path periodically to correct the detector. Some systems have permanent multi-point non-uniformity correction, where the detector is corrected at a variety of scene temperatures for each range and then the data is stored within the unit, so the user never has to perform a non-uniformity correction in the field. This appears to be the best approach since it requires no user intervention and also provides for non-uniformity correction at several temperatures and not just at the lens cap temperature as with other approaches. Non-uniformity correction is an important parameter for the P/PM user to consider given that it needs to be done each time you change ranges, lenses, or when the camera operating temperature varies. Systems that do this automatically will prove to be the easiest to use in the field. The best non-uniformity correction will be accomplished at a temperature as close to the object temperature as possible.

Optical and Digital Zoom

Optical magnification is directly related to the lens’ focal length; it makes the lens move inwards or outwards to get a better view of an object, magnifying it. Zooming in with an optical zoom results in a smaller angle of view, but retains image quality. Digital Zoom, on the other hand, is completely comprised of pixels. When digital zoom is used, it digitally enlarges the pixels of an image which results in a blurrier (more noticeably “pixelated”) image when compared to Optical Zoom.

Pixel

Contraction of “picture element”. A small element of a scene, often the smallest resolvable area, in which an average brightness value is determined and used to represent that portion of the image. Pixels are arranged in a rectangular array to form a complete image.

Refresh Rate

See Framerate / Frequency.

Thermal sensitivity

The quantitative description of how well a thermal device distinguishes temperature differences. It is expressed in mK (milliKelvin or one-thousandth of a Kelvin). A lower numerical value (in mK) indicates higher sensitivity, because the device can discern smaller differences in temperature. Typical value of FPA sensitivity is 50mK, which means that the detector can differentiate objects if the difference in their surface temperatures is 50mK or more, e.g. 11.95°C and 12°C.

Thermoelectric Cooling

A refrigeration method based on the Peltier effect. When an electric current passes through a thermocouple of two dissimilar metals joined in two places, heat is absorbed at the cold junction and dissipated at the hot junction. The cold junction can be mounted in the chamber to be cooled.